1. Field of the Invention
This invention is intended to improve a signal-to-noise ratio (S/N) of an infrared image pickup device, and to reduce variations of an output offset level due to temperature variations of the infrared image pickup device.
2. Description of the Prior Art
FIG. 1 of the accompanying drawings is a block diagram showing one example of an existing infrared image pickup device. The infrared image pickup device comprises: bolometers 1-4; pixel separating diodes 5-8 whose anodes are connected to the bolometers 1-4; horizontal FET switches 9 and 10 whose sources are connected to the bolometers 1-4; a horizontal scanning circuit 11 connected to gates G of the horizontal FET switches 9 and 10; a bias current output terminal 12; vertical FET switches 13 and 14 whose drains D are connected to cathodes of the pixel separating diodes 5-8 and whose sources S are connected to the bias current output terminal 12; a vertical scanning circuit 15 connected to gates G of the vertical FET switches 13 and 14; a bias current input terminal 16; a fixed load resistor 17 which is connected between drains D of the horizontal FET switches 9 and 10, and the bias current input terminal 16; an amplifier 18 whose input terminal is connected to a node between drains D of the horizontal FET switches 9 and 10, and the fixed load resistor 17; and an output terminal 19 of the amplifier 18. All of the foregoing elements are mounted on a substrate 20 made of silicon or the like. In order to simplify the description, FIG. 1 shows the infrared image pickup device having (2 2) pixels. A resistance value of the fixed load resistor 17 is set to a value which is substantially equal to a sum of resistance values of the bolometers 1-4 and conductive resistance values of the vertical FET switches 13 and 14, and those of the horizontal FET switches 9 and 10. FIG. 2 is a block diagram of an infrared camera adopting an infrared image pickup device. This camera comprises an infrared optical system 21, a substrate 20 (for mounting the infrared image pickup device), a Peltier element 22 heat-conductively contacted to the substrate 20, and a device driving and display processing circuit 23 which is connected to the horizontal scanning circuit 11, the vertical scanning circuit 15, the bias current input terminal 16, the bias current output terminal 12, and an output terminal 19. In operation, the substrate 20 is first controlled to a predetermined temperature using the Peltier element 22. The device driving and display processing circuit 23 provides clock signals to the horizontal scanning circuit 11 and the vertical scanning circuit 15. The device driving and display processing circuit 23 also applies a bias voltage between the bias current input terminal 16 and the bias current output terminal 12.
The infrared optical system 21 converges infrared rays radiated by an object to be imaged onto the substrate 20 where the bolometers 1-4 are mounted. The bolometers 1-4 absorb the infrared rays, and minute temperature rises are caused in the bolometers 1-4 in proportion to the intensities of the infrared rays. The bolometers 1-4 are made of materials such as polysilicon or vanadium oxide whose resistance extensively varies with temperature. The bolometers 1-4 change their own resistance values in accordance with the temperature rise.
Next, the vertical scanning circuit 15 provides clock signals to the vertical FET switches 13 and 14 so that the vertical FET terminal's 13 and 14 sequentially become active. The horizontal scanning circuit 11 provides clock signals to the horizontal FET switches 9 and 10 so that the horizontal FET switches 9 and 10 sequentially become active.
For example, in order to read a resistance value of the bolometer 1, the vertical FET switch 13 and the horizontal FET switch 9 will be turned on. In this case, a current sequentially flows through the bias current input terminal 16, fixed load resistor 17, horizontal FET switch 9, bolometer 1, pixel separating diode 5, vertical FET switch 13, and bias current output terminal 12. In this state, there is a bypass between the bolometer 3, pixel separating diode 7, pixel separating diode 8, bolometer 4, bolometer 2, and pixel separating diode 6, in parallel to a main path. However, this bypass is electrically disconnected, since the pixel separating diode 8 is reverse biased. Thus, a desired pixel can be selectively read out. In order to selectively read a resistance value of the bolometer 2, 3 or 4, the pixel separating diode 7, 6 or 5 will become reverse biased.
FIG. 3 shows the relationship between read timing of the infrared image pickup device and a voltage to be applied to the amplifier 18. Signal voltages, whose levels are approximately 50% of a bias voltage and which correspond to the resistance values of the respective bolometers 1-4, minutely vary with an amount of incident infrared rays. However, while both the horizontal FET switches 9 and 10 remain inactive, the voltage applied to the amplifier 18 is raised until it becomes equal to the bias voltage. The amplifier 18 amplifies this voltage, and outputs it via the output terminal 19.
In this conventional infrared image pickup device, the signal voltage, which is applied to the amplifier 18 while either the horizontal FET switch 9 or 10 is active and whose level is approximately 50% of the bias voltage, minutely varies with the amount of infrared rays. On the other hand, while both the horizontal FET switches 9, 10 remain inactive, the signal voltage applied to the amplifier 18 is increased to the bias voltage. A difference between the levels of the signal voltages, which are applied during the on-state of the horizontal FET switch 9 or 10, and during the off-state of both the horizontal FET switches 9 and 10, is very large compared with a signal amplitude depending upon intensities of the incident infrared rays. Thus, it is necessary to increase a frequency band width of the device driving and display processing circuit 23 above a frequency band necessary for reading, thereby improving transient response characteristics. However, this disadvantageously increases noise, and reduces a signal-to-noise ratio. Further, the resistances of the bolometers 1-4, and conductive resistances of the vertical and horizontal FET switches 13, 14, 9 and 10 vary with temperature. Although the infrared image pickup device includes a mechanism for controlling the temperature of the substrate 20, offset variations, which are larger than the signal output caused by variations of infrared rays, will be outputted if the temperature of the substrate 20 changes with disturbances such as a variation of ambient temperature.